Thioglycerol-Substituted Zinc(II) Phthalocyanines as Antibacterial Agents against Staphylococcus aureus and Escherichia coli

The growing concern surrounding antibiotic resistance led to the development of new therapeutic alternatives, among which antimicrobial photodynamic therapy (aPDT) has emerged as a promising approach. This technique requires the presence of oxygen, light and a photosensitizer (PS), in order to produce reactive oxygen species (ROS), responsible for the inactivation of bacteria. Among all the well-known types of PSs, phthalocyanines are among the most promising ones, due to their intense absorption and strong optical properties (e.g., high synglet oxygen quantum yields (Φ_Δ) and photostability). Even so, aggregation is still a drawback that needs to be overcome in these PSs. Thus, the insertion of suitable functional groups in the peripheral positions is demanding to enhance biocompatibility.

This study suggests the use of thioglycerol-containing zinc(II) phthalocyanines (ZnPcs) as PSs to photoinactivate Gram-positive bacteria (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli). The two presented PSs (ZnPcs 1 and 2) were synthesized using microwave assistance, from the derivatized phthalonitriles, and their structures were fully characterized. Their optical properties were studied, which showed high solubility in phosphate buffer saline (PBS) solution, high Φ_Δ (ZnPc 1, Φ_Δ = 0.50; ZnPc 2, Φ_Δ = 0.55) and photostability.

In conclusion, these photophysical results showed that the designed phthalocyanines are promising candidates for the aPDT activity against E. coli and S. aureus. Therefore, further in vitro studies will be conducted to evaluate the efficacy of the PSs to photoinactivate the two types of bacteria.